Marcelo Navarro

Two convenient and high-yielding preparations of 6,6′-dimethyl-2,2′-bipyridine by homocoupling of 6-bromopicoline

Abstract 6-Bromopicoline can be reductively homocoupled to provide 6,6′-dimethyl-2,2′-bipyridine in high yield either by an Ni·bpy catalysed electrosynthesis or by a catalytic modification of the Ullmann synthesis employing Pd(OAc) 2 . Either procedure is operationally simple and represents a considerable improvement over most known syntheses of this important disubstituted bipyridine.

Electrochemical synthesis of TGA-capped CdTe and CdSe quantum dots

The electrochemical preparation of Te2− and Se2− ions was carried out in NaOH aqueous solution, with high yield and good stability, and applied in the preparation of negatively charged CdTe and CdSe quantum dots (QDs). A CdCl2 aqueous solution containing thioglycolic acid (TGA) stabilizer was mixed with the electrochemically generated Te2− or Se2− ions in a quite reproducible and clean one pot process, without the use of chemical reducing agents. The CdTe and CdSe QDs had good stability and high luminescence with an average size of d = 3.9 nm and 2.6 nm, respectively, the quantum yields and lifetimes were also determined. Hg2+ was efficiently quantified in the presence of the synthesized TGA-capped CdTe and CdSe, making possible the application of these QDs as chemical sensors.

Aqueous Barbier allylation of aldehydes mediated by tin.

The aqueous tin-mediated Barbier reaction affords good to excellent yields and moderate syn diastereoselectivity under basic and acidic conditions. The high yields and stereoselectivity observed in the case of o-substituted aldehydes suggest a cyclic organotin intermediate or transition state in K2HPO4 solution. A practical and efficient aqueous tin allylation of methoxy- and hydroxybenzaldehydes can be carried out in HCl solution in 15 minutes to afford the corresponding homoallylic alcohols in high yields. Aliphatic aldehydes give moderate to excellent yields with reaction times ranging from 30 to 60 minutes. Under these conditions, crotylation gives exclusively the gamma-product and the syn isomer is formed preferentially. For 2-methoxybenzaldehyde, an equilibration of the isomers to a syn/anti ratio of 1:1 can be observed after several hours. Control experiments with radical sources or scavengers give no support for radical intermediates. NMR studies suggest a mechanism involving an organotin intermediate. The major organotin species formed depends on the reaction medium and the reaction time. The use of acidic solution reduces the reaction times, due to the acceleration of the formation of the allyltin(IV) species.

Metals in the sediments of Jauá Lake (Camaçari, Bahia, Brazil) following an episode of industrial contamination

Abstract The contamination of sediments caused by the deposition of industrial residues from titanium dioxide production, in sand dunes near a wetland was assessed through atomic absorption spectrometry. The contamination occurred near a shallow freshwater wetland called Jaua Lake, along the coast of Camacari, Bahia, Brazil. Five core samples were collected, including a reference site, from Jaua Lake and one from a small lake, near the deposition site. Cores were cut in 20-cm sections. Fractions <63 μm were analysed for copper, cadmium, zinc, iron, lead, aluminium, mercury and titanium. Metal concentrations on the upper layer of sediments were, as a whole, higher than in lower ones. Concentrations from the reference site were similar to those from the other sites in Jaua Lake. Absolute values of most metals in the sediments of the Dunas Lake located near the contamination site were higher than in all other stations. The hypothesis, that:: (1) contamination coming from groundwater would contaminate the sed...

Electrochemical synthetic route for preparation of CdTe quantum-dots stabilized by positively or negatively charged ligands

An electrochemical route for the generation of Te2− ions in an aqueous solution, with high yield and great stability, was developed and applied for the preparation of CdTe quantum dots (QDs) stabilized by negatively or positively charged ligands. An aqueous Cd(ClO4)2 solution containing mercaptopropionic acid (MPA) or cysteamine (Cys) stabilizers was directly added to the electrochemically generated aqueous telluride ion solution. This one pot process is quite reproducible and clean (no chemical reducing agents), allowing control of reduced species generation. The CdTe QDs showed good stability and high luminescence with particle average sizes of d = 3.1 nm (MPA) or 2.7 nm (Cys). Macrophages were efficiently labeled with MPA-capped CdTe QDs as well as with Cys-capped CdTe QDs.

Electrochemical allylation of aldehydes in a solvent-free cavity cell with a graphite powder cathode

Mixtures of carbonyl compounds and allyl halides, adsorbed on a graphite powder cathode, were reduced under aqueous anolyte to give high yields of homoallylic alcohols. Catalytic amounts of silver or tetrabutylammonium tetrafluoroborate improved the yields significantly. Inexpensive allyl chloride gave superior results when compared to allyl bromide.

Electrocatalytic hydrogenation of organic compounds using current density gradient and sacrificial anode of nickel

Abstract Preparative electrocatalytic hydrogenation (ECH) of some organic compounds were performed: cyclohexene, 2-cyclohexen-1-one, benzaldehyde, acetophenone, styrene, 1,3-cyclohexadiene, trans – trans -2,4-hexadien-1-ol, citral, linalool and geraniol. H 2 O/MeOH (1:1), NH 4 OAc or NH 4 Cl (0.2 M) were used as solvent and supporting electrolyte. A sacrificial anode of nickel allowed the use of an undivided cell, with a cell voltage varying between 2.3 and 1.3 V, depending on the supporting electrolyte. A current density gradient was applied to diminish the time of reaction and obtain a good electrochemical efficiency. An in situ prepared cathode of nickel deposited on iron provided a highly efficient ECH process, and the constant deposition of nickel on the electrode surface avoided catalyst poisoning. The ECH system was somewhat selective, hydrogenating conjugated olefins in good yield.

Metal-free electrochemical Reformatsky reaction in water: further evidence for a radical mechanism

Abstract The first report of an electrochemical Reformatsky reaction in aqueous medium and in the absence of metal is presented. Ethyl bromoisobutyrate ( 1 ) and benzaldehyde ( 2 ) were electrolyzed in a divided cell at a potential of −1.2 V versus Ag/AgCl, corresponding to the reduction wave of 1 . Several types of carbon cathodes and a platinum anode were used in a 0.1 M KBr solution in either water+methanol (5:1) or water. The Reformatsky adduct ethyl 2,2-dimethyl-3-hydroxy-3-phenyl propionate ( 5 ) was obtained with yields up to 46% (relative to consumed 2 ) together with varying amounts of ethyl isobutyrate ( 3 ) and diethyl 2,2,3,3-tetramethylsuccinate ( 4 ). The probable mechanism starts with a radical species generated from 1 which adds to benzaldehyde giving a radical intermediate, which is further reduced to the final product 5 . A radical chain through bromine abstraction from 1 is also possible. No products resulting from reduction of benzaldehyde were observed. On zinc electrodes, in an undivided cell, an organometallic reaction producing the same coupling product 5 becomes more important. The results represent a strong additional support for a radical mechanism postulated in previous work for the Zn-promoted chemical Reformatsky reaction in water.

Rehabilitation Following Industrial Contamination: Jauá Lake, a Coastal Wetland in Camaçari, Bahia, Brazil

In North America and Europe, there is a broad consensus among scientists, the public, and politicians on the importance of wetlands. As a direct consequence wetlands are well studied and laws provide some protection for these ecosystems. However, in the tropical zone of South America, wetlands are seen as unusable lands and nuisance sites that support disease vectors. Therefore, neotropical wetlands have been subjected to severe pressures since the early years of colonization by Europeans. Although wetlands are a conspicuous part of the landscape in South America and floodplains alone cover many hundreds of thousands of square kilometers (Sippel et al. 1992, Junk 1993), little is known about these systems. Only one wetland study (Junk and da Silva 1995) is presented in Tundisi et al.’ s (1995) recent review of limnology in Brazil, South America’ s largest country. More optimistic though is the fact that during the XXVI Congress of the International Association of Theoretical and Applied Limnology, held in Sao Paulo (SIL 1995), thirty-two presentations out of 830 dealt with the question of neotropical wetlands.

Hidrogenação de compostos orgânicos utilizando método eletroquímico para geração de hidrogênio in situ: hidrogenação eletrocatalítica

Electrocatalytic hydrogenation (HEC) may be compared to catalytic hydrogenation (HC). The difference between these methods is the hydrogen source: HC needs a hydrogen gas supply; HEC needs a source of protons (solvent) to be reduced at a cathode surface. HEC has presented interesting advances in the last decades due to investigation of the influence of the supporting electrolyte, co-solvent, surfactant, presence of inert gas and the composition of the electrode on the reaction. Several classes of organic compounds have been hydrogenated through HEC: olefins, ketones, aldehydes, aromatics, polyaromatics and nitro-compounds. This paper shows some details about the HEC which may be regarded as a promising technique for the hydrogenation of organic compounds both in industrial processes and in laboratories.